1. Field of the Invention
The present invention relates to a base station apparatus and radio communication method used in a digital radio communication system.
2. Description of the Related Art
A propagation model in a radio communication is explained with reference to FIG. 1. By way of example, suppose the number of antennas of the radio communication apparatus (base station apparatus) is 3. In FIG. 1, two paths A and B indicate downlink (transmission from a base station to terminal) propagation paths. A signal transmitted from base station apparatus 1 is reflected by building 2 and arrives at an antenna of terminal apparatus 3. Such a propagation path is called a xe2x80x9cmulti-path propagation pathxe2x80x9d and communication quality generally deteriorates if this multi-path propagation cannot be compensated. In this example, suppose the signal from building 2 is received by the receiving side with a delay within the range of its time resolution. Transmission directivity in this case is shown in FIG. 2.
Thus, if this signal contains a great delay, this delay may be a major factor of deterioration of the communication quality. To suppress multi-path propagation, it is desirable to transmit signals to either path A or path B. Furthermore, a communication system such as a CDMA transmission system in which a same band and time are shared can suppress interference with other stations by narrowing the range of transmission directivity, providing an effective way of achieving high frequency utilization efficiency. Therefore, it is extremely important to detect a direction of an optimal communication quality and perform transmissions focusing on that direction.
FIG. 3A to FIG. 3C are delay profiles showing the propagation path characteristics of path A and path B in FIG. 1. In FIG. 3, the horizontal axis represents the time and the vertical axis-represents the propagation loss. That is, t0 and t1 on the receiving side represent timings of path A and path B, respectively and the height difference represents a difference in the reception level (difference in propagation loss). The fact that the reception timing differs between path A and path B means that path A and path B differ in the propagation distance.
A delay profile generally changes as a terminal moves. That is, the communication quality of each of path A and path B changes. FIG. 3A shows that the communication quality of path A is better, while FIG. 3B shows that both paths have equivalent levels of communication quality and FIG. 3C shows that the communication quality of path B is better.
A conventional base station apparatus is explained below. FIG. 4 is a block diagram showing a configuration of a base station apparatus that carries out conventional adaptive array transmission. By way of example, suppose the number of antennas is 3.
The transmitting side of this terminal modulates a transmission signal by modulation circuit 11. A plurality of reception weighting factors calculated by weighting factor calculation circuit 12 based on an advance information signal are output to selection circuit 13 where an optimal weighting factor is selected, and processing circuit 14 performs a multiplication (generally complex multiplication) using this weighting factor. Naturally, it is also possible to perform a multiplication after calculating only an optimal weighting factor. Then, radio transmission circuit 15 carries out frequency conversion and amplification on the transmission signal and transmits it from antennas.
In a propagation environment as shown in the delay profiles in FIG. 3, the base station apparatus above performs transmission by forming directivity in the direction of path A when the communication quality of path A is better as shown in FIG. 3A. The base station apparatus also performs transmission by forming directivity in the direction of path B when the communication quality of path B is better as shown in FIG. 3C. On the other hand, if path A and path B have equivalent levels of communication quality as shown in FIG. 3B, the base station apparatus performs transmission by forming directivity in either direction.
Therefore, if the other end of communication is a moving terminal, the delay profiles change with time, and therefore, the base station apparatus shown in FIG. 4 can always perform transmission with array antennas of an optimal communication quality by making its weighting factor selection circuit switch a weighting factor according to a change in the delay profiles.
Here, transmission timing is generally not changed in accordance with directivity switching. This is because in the case of continuous transmission, changing transmission timing will cause problems such as discontinuation or overlap of a transmission signal and collapse of orthogonality (code orthogonality in the case of CDMA, and time orthogonality in the case of TDMA) with other channels with which the transmission signal is multiplexed. etc.
The calculation of the receiving side at the other end of communication (terminal) is explained using FIG. 5. On the terminal side, a reception signal received by an antenna is output to radio reception circuit 22 via antenna duplexer 21. Radio reception circuit 22 carries out amplification, frequency conversion and A/D conversion on the reception signal and extracts a baseband signal or IF signal.
In a CDMA system using a spread spectrum (SS) communication system, a reception signal is output to correlator (or matched filter) 23 and despread by the same spreading code as that used for spreading processing on the transmitting side. The despread signal is output to timing detection circuit 24. Timing detection circuit 24 calculates the power of the correlator output and detects time to when the power is large. This timing to is output to sampling circuit 25. Sampling circuit sends the reception signal with timing t0 to demodulation circuit 26. Demodulation circuit 26 demodulates and outputs the reception signal.
On the other hand, a non-CDMA communication system generally sends the extracted baseband signal or IF signal to timing detection circuit 24. Timing detection circuit 24 calculates an optimal reception timing. The optimal reception timing is calculated, for example, by the transmitter side embedding a pattern known to both the transmitter and receiver in a frame and transmitting this signal. The receiver side performs A/D conversion with several to over ten times the one-symbol time and performs a correlation calculation with the known symbol. Then, the receiver side detects timing to when the power resulting from the correlation calculation is large. This timing to is output to sampling circuit 25. Sampling circuit 25 sends the reception signal of timing t0 to demodulation circuit 26. Demodulation circuit 26 demodulates and outputs the reception signal.
On the other hand, the transmission signal is modulated by modulation circuit 27, that is, in the CDMA transmission system, the transmission signal is spread using a predetermined spreading code. The modulated signal is frequency-converted and amplified by radio transmission circuit 28 and transmitted from an antenna via duplexer 21.
Then, the following is an explanation of the calculation of a base station in a radio communication system when adaptive array reception and adaptive array transmission based on information thereof are applied. The calculations of the conventional base station in FIG. 6 and the terminal in FIG. 5 are explained. By way of example, suppose the number of antennas of the apparatus is 3.
First, the uplink is explained. The terminal on its transmitting side modulates a transmission signal by modulation circuit 27. This modulated signal is frequency-converted and amplified by radio transmission circuit 28 and transmitted from the antennas via antenna duplexer 21.
The base station sends signals received from its antennas to radio reception circuit 32 via respective antenna duplexers 31. Radio reception circuit 32 carries out amplification, frequency conversion and A/D conversion on the reception signals and extracts baseband signals or IF signals. If the transmission and reception signals have the same frequency (TDD transmission), changeover switches are used instead of duplexers. These signals are output to timing detection circuit 34.
Timing detection circuit 34 calculates an optimal reception timing. The optimal reception timing is calculated, for example, by embedding a pattern known to both a transmitter and receiver in a frame and transmitting this signal from the transmitter. The receiver performs A/D conversion with several to over ten times the one-symbol time and performs a correlation calculation with the known symbol. Then, the receiver detects timing t0 when the power resulting from the correlation calculation is large. This timing t0 is output to sampling circuit 35.
Sampling circuit 35 sends the reception signal with timing t0 to adaptive array antenna reception circuit 36. Adaptive array antenna reception circuit 36 combines the reception signals from the three antennas so that a desired wave or SIR reaches a maximum value for each timing. Then, adaptive array antenna reception circuit 36 outputs the reception signals and reception weighting factors to be multiplied on the reception signals of the respective antennas. These weighting factors form reception directivity.
If adaptive array antenna processing is performed so as to extract a desired signal, the directivity is directed to the desired signal, generating a part with small directivity (called xe2x80x9cnullxe2x80x9d) in an unnecessary signal (signal identical to the desired signal, which arrives at a different time because the propagation path is different, or signal from another transmitter). The number of null points is known to be (the number of array antennas xe2x88x921) and if the number of antennas is 3, 2 null points are formed.
In the case of a CDMA system using a spread spectrum (SS) communication system, correlator (or matched filter) 33 performs despreading using the same spreading code as that used for spreading processing for baseband signals or IF signals on the transmitter side. The despread signals are output to timing detection circuit 34. Timing detection circuit 34 calculates the power of the correlator output and detects times t0 and t1 when the power is large. These timings t0 and t1 are output to sampling circuit 35.
Sampling circuit 35 sends the reception signals with timings t0 and t1 to adaptive array antenna reception circuit 36. Adaptive array antenna reception circuit 36 combines the reception signals from the three antennas so that a desired wave or SIR reaches a maximum value for each of reception timings t0 and t1 using the weighting factors calculated by reception weighting factor calculation circuit 37 and finally combines additional reception signals corresponding to 2 paths. Then, adaptive array antenna reception circuit 36 outputs the resultant reception signals and two reception weighting factor sets to be multiplied on the reception signals of the respective antennas. These two weighting factor sets form reception directivities with reception timings t0 and t1, respectively.
Then, the downlink is explained. The base station modulates a transmission signal by modulation circuit 38. Transmission weighting factor calculation circuit 39 regenerates transmission weighting factors based on the reception weighting factors. Then, processing circuit 40 performs a multiplication (generally complex multiplication) by an optimal transmission weighting factor after selecting a weighting factor set by selection circuit 41. At this time, as explained in the calculation of the base station shown in FIG. 4, weighting factor selection circuit 41 can always perform transmission with array antennas of an optimal communication quality by switching between the two transmission weighting factor sets according to changes in the delay profiles.
As shown above, transmitting signals with the same directivity pattern as the reception directivity pattern based on the weighting factors of the reception signals combined through an adaptive array antenna prevents signals being transmitted in directions of unnecessary signals that have arrived, and therefore allows the transmitting side to compensate the multi-path propagation path. This eliminates the need for providing the receiver (terminal side) with high-class devices such as equalizer.
Avoiding transmitting signals in directions of unnecessary signals that have arrived limits the reach of radio waves transmitted, improving thus the downlink frequency utilization efficiency. Since transmission is also performed via a propagation path with the desired wave power on the uplink or with great SIR taking advantage of reversibility of the propagation path, the desired wave power or SIR increases on the downlink as well.
However, when the switching of transmission directivities is controlled according to the conventional system described above, if transmission is performed by selecting a path with a widely different propagation delay, the reception timing on the receiving side suddenly changes, which causes problems such as preventing it from receiving signals correctly until the receiving side detects a new reception timing and switches directivities or causing instantaneous interruption of a reception signal due to loss of synchronism.
In a CDMA system using spread spectrum communications in particular, when the switching of transmission directivities is controlled using reception directivities through a reception adaptive array, if directivity transmission is performed by selecting a path with a widely different propagation delay, the propagation path suddenly changes, which causes problems such as preventing the terminal from receiving signals correctly because a search and finger assignment cannot follow the change or causing instantaneous interruption of a reception signal due to loss of synchronism.
It is an objective of the present invention to provide a base station apparatus and radio communication method capable of receiving a signal correctly and preventing instantaneous interruption due to loss of synchronism when the switching of transmission directivities is controlled and even when transmission is performed by selecting a path with a widely different propagation delay.
The present inventor has come up with the present invention by discovering that it is possible to solve the problems of the propagation path suddenly changing preventing the terminal from receiving signals correctly because a search and finger assignment cannot follow the change or loss of synchronism even if adaptive array transmission is performed by selecting a path with a greatly different propagation delay.
That is, when selecting a path where a directivity changes greatly, the present invention performs transmission in both directivities like soft handover and the terminal performs reception by combining them. Then, the terminal switches to one directivity according to the reception level. In this specification, this technology is called xe2x80x9cpath handover (PHO).xe2x80x9d
This PHO is especially effective for directivity switching when introducing an adaptive array antenna for the downlink of a W-CDMA system. PHO makes it possible to improve the reception characteristic on the terminal and prevent instantaneous interruption of a reception.